let p, q, r, s be Element of CQC-WFF ; :: thesis: ( p => q in TAUT & r => s in TAUT implies (p 'or' r) => (q 'or' s) in TAUT )
assume ( p => q in TAUT & r => s in TAUT ) ; :: thesis: (p 'or' r) => (q 'or' s) in TAUT
then ( (p 'or' r) => (q 'or' r) in TAUT & (q 'or' r) => (q 'or' s) in TAUT ) by Lm6, Lm7;
hence (p 'or' r) => (q 'or' s) in TAUT by LUKASI_1:3; :: thesis: verum